Currently, no data exists regarding the safety and tolerability of a combination regimen utilizing weekly topotecan in combination with pemetrexed. Although both drugs are associated with myelosuppression, it is hoped that the utilization of the weekly topotecan dosing schedule will allow the drugs to be easily combined. This phase I trial will evaluate the safety and tolerability of weekly topotecan in combination with pemetrexed in patients with advanced solid tumors.